Bad error message for arguments between ParamSpec P.args and P.kwargs

[JonathanPlasse]

Bug Report

ParamSpec P.args and P.kwargs are not defined when using explicit keyword argument.

To Reproduce

This code would be used to type anyio.run().

Link to mypy-play

from typing import ParamSpec, Awaitable, TypeVar, Callable, Any

P = ParamSpec("P")
T_Retval = TypeVar("T_Retval")

def run(
    func: Callable[P, Awaitable[T_Retval]],
    *args: P.args,
    backend: str = "asyncio",
    backend_options: dict[str, Any] | None = None,
    **kwargs: P.kwargs,
) -> T_Retval:
    pass

but it works without explicit keyword arguments.

from typing import ParamSpec, Awaitable, TypeVar, Callable, Any

P = ParamSpec("P")
T_Retval = TypeVar("T_Retval")

def run(
    func: Callable[P, Awaitable[T_Retval]],
    *args: P.args,
    **kwargs: P.kwargs,
) -> T_Retval:
    pass

Expected Behavior

It should be able to type and recognize that run() passes the rest of the arguments to the function a().

async def a(b: str, c: int) -> None:
    pass

run(a, "", c=0)

Actual Behavior

main.py:10: error: Name "P.args" is not defined
main.py:13: error: Name "P.kwargs" is not defined

Your Environment

• Mypy version used: 0.981
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

[erictraut]

With ParamSpec, you are not allowed to define parameters between *args and **kwargs. PEP 612 is very clear on this point:

Placing keyword-only parameters between the *args and **kwargs is forbidden.

Mypy's error message in this case should probably be improved to indicate as such.

If you move the parameters backend and backend_options before *args, it will type check without errors. That assumes you want backend and backend_options to be positional + keyword parameters and not keyword-only parameters. There's no way to combine ParamSpec with keyword-only parameters because of the constraint I mentioned above.

[hauntsaninja]

mypy 0.990 will have a less atrocious error message here (but definitely could still be made more helpful)

[ilevkivskyi]

Yeah, the error message is still not very good:

ParamSpec must have "*args" typed as "P.args" and "**kwargs" typed as "P.kwargs"